Comparison of serotonin 5‐HT 1 receptors and innervation in the visual cortex of normal and dark‐reared cats

The visual cortical serotoninergic system was compared in normal and dark‐reared cats to determine whether visual experience is necessary for its normal development. In vitro receptor binding of [ 3 H]5‐HT indicated an increase in 5‐HT 1 receptor number in dark‐reared cats with no change in affinity. This elevation was specific to the visual cortex and no changes were found in the frontal cortex as a result of dark rearing. Autoradiographic histology revealed that in the normal cat visual cortex, 5‐HT 1 receptors were present in all cortical layers and were slightly more dense in supragranular and infragranular layers. In dark‐reared cats, there was a marked elevation in receptor density in supragranular and infragranular layers and little change within layer IV. Immunohistochemical techniques (anti‐5‐HT) were used to compare serotoninergic innervation in the visual cortex of normal and dark‐reared cats. In normal cat visual cortex, serotonin fibers were most dense in the superficial layers (I‐III), least dense in layers IV and VI, and intermediate in layer V. No differences were found between normal and dark‐reared cats in the laminar distribution or density of serotoninergic innervation. These results indicate that visual experience is necessary for the normal development of the visual cortical serotonin system. The findings that the effects of dark rearing were specific to the visual cortex and that within the visual cortex these effects were specific to supra‐ and infragranular layers are consistent with a possible role for serotonin in the prolonged physiological plasticity that occurs in the visual cortex of dark‐reared cats.